1. Field of the Invention
The present invention relates to a radiation sensor, which is used in micro-radiation detecting apparatuses for the purpose of analyzing the radiation energy spectrum, diagnostic X-ray radiographic apparatuses, non-destructive inspection apparatuses, bone densitometer, and the like. The present invention also relates to a radiation detecting apparatus using the radiation sensor.
2. Description of the Prior Art
FIG. 26 shows a conventional semiconductor radiation sensor of totally depleted layer type, in which on the upper surface and the back surface of a semiconductor substrate 1, a split electrode 2 and a common electrode 3 are provided, respectively. The electric charges generated in the semiconductor substrate 1 by radiation 4 are detected by the application of a voltage between the electrodes 2 and 3. According to Ramo's theorem, a current produced by the movement of the electric charges is given as follows: EQU i=q.multidot.dx/dt (I)
where q is the amount of electric charge generated and dx/dt is the drift velocity of the electric charge.
For the purpose of energy spectrum analysis of radiation, a current i in Equation I is integrated to obtain the amount of electric charge generated as follows: EQU Q=.intg.idt (II)
Thus, the output current from the detector, which is due to electrons and positive holes, is converted to the amount of electric charge by means of an integration circuit of an external measuring instrument, resulting in a measured value.
However, in cases where an external measuring instrument which utilizes an integration circuit is used, although the height of an output pulse is proportional to the photon energy of the incident radiation when the radiation incident upon the detector includes a small number of photons, miscount of photons will occur when the incident radiation includes an increased number of photons and also the incident rate is shorter than the time constant of the integration circuit used.
To solve such a problem, some measurement systems use a high-speed pulse amplifier in place of the integration circuit. Such measurement systems work satisfactorily when electrons and positive holes have a similar mobility (i.e., drift velocity of electric charge in a constant electric field) such as in cases where the substrate is made of Si or Ge. However, when electrons and positive holes have different mobilities as in cases where the substrate is made of a compound semiconductor such as GaAs, CdTe, CdSe, or HgI.sub.2, the following problems will arise. When the measurement of current produced by either of electric charges (i.e., electrons or positive holes) is attempted, because the electric charges generated in the vicinity of a collector electrode have a very short transit time, it results in a very low gain when the electric charges pass the pulse amplifier as a current pulse. Consequently, the height of the output current pulse from the pulse amplifier depends on the position at which the electric charges are generated.
Moreover, when both electrodes are arranged parallel to the incident direction of radiation, mounting of a sensor becomes difficult.